Many radio communication systems use methods of controlling transmission power aimed at reducing the level of interference between different communications. Such control of power is of particular importance in spread spectrum systems using Code Division Multiple Access (CDMA). In such systems, several terminals can share the same frequency at each moment, the separation of the channels over the radio interface resulting from the quasi orthogonal nature of the spread codes respectively applied to the signals transmitted over those channels. In other terms, for a given channel, the contributions of the other channels are regarded as noise.
In particular, on the uplink, control of transmission power limits the transmission power of mobiles close to a base station to prevent the signals they are transmitting from masking the signals from mobiles further away. In general, the methods of controlling power use power control loops: the base station takes measurements on the signal received from a mobile (power, carrier-to-interference ratio (C/I), etc) and transmits commands to increase or reduce power on the downlink to tend toward a given quality objective. These methods cannot be used before a radio link is established between the base station and the mobile. In particular, they do not allow idle mobiles to determine the level of power at which they must transmit any random access requests.
With UMTS (Universal Mobile Telecommunications System) systems, the loops controlling transmission power on the uplink are described in technical specification 3G TS 25.401, version 3.3.0, published in June 2000 by the 3GPP (3rd Generation Partnership Project), pages 20–21.
For the power of the first signals transmitted by a mobile terminal to a base station, particularly to set up a new call, these power control loops are not operational, because the base station has not received any prior signal from the mobile terminal with which to take the required measurements. The mobile terminal then estimates the power of these first signals according to another procedure based on the attenuation of the signals transmitted by the base station and received by the mobile terminal. The base station broadcasts a marker signal indicating the power at which it has transmitted the said signal. The receipt of such marker signals enables the idle mobile to determine the resources used by the base station with which the link is best (cell selection) and to evaluate the attenuation of the signal from that station. From this it deduces an initial transmission power for the radio signals sent to the selected base station, and the greater the attenuation the greater the power.
In certain circumstances, particularly when the mobile terminal is very close to the receiving antenna of the base station, the result of this estimate can be a very low transmission power. Such may be the case, for example, with a call from a maintenance agent working on the base station itself and using his radio terminal.
Because of its construction, a radio terminal has a minimum radio transmission power below which it is not capable of transmitting. Technical specification 3G TS 25.101, version 3.6.0, published in March 2001 by the 3GPP, recommends a minimum transmission power by UMTS mobile terminals of −50 dBm (section 6.4.3, page 13).
If the transmission power estimated for the random access request is less than this minimum power, the mobile terminal transmits the random access request with its minimum transmission power (see technical specification 3G TS 25.214, version 3.6.0, published by the 3GPP in March 2001, section 6.1, page 27).
If this transmission power is significantly greater than the estimated power based on the attenuation measurements, said transmission may generate consequent noise for the other radio signals received by the base station and therefore adversely affect the transmission quality of the calls in progress to which those other signals belong.
A purpose of the present invention is to prevent such interference that one call can cause to all the others.